Enable black

- blackify all the python files,
- enable black in pre-commit,
- add a black tox environment.

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-envlist=flake8,mypy,py3
+envlist=black,flake8,mypy,py3
 
 [testenv]
 extras =
@@ -13,6 +13,13 @@ commands =
   !dev: --cov={envsitepackagesdir}/swh/lister/ --cov-branch \
          {envsitepackagesdir}/swh/lister/ {posargs}
 
+[testenv:black]
+skip_install = true
+deps =
+  black
+commands =
+  {envpython} -m black --check swh
+
 [testenv:flake8]
 skip_install = true
 deps =